Variable resistance equalizer



Feb. 6, 1945. THOMPSON 2,369,088

VARIABLE RESISTANCE EQUALIZER Filed July 20, 1943 5 SheerLs-Sheet l REPRODUCER AMPLIFIER 9378030 INVENTOR LINCOLN THOMPSON BY ATTORNEY Feb. 6, 1 945. fi-HOMPSON I 2,369,088

VARIABLE RESISTANCE EQUALIZER Filed July 20, 1943 5 Sheets-Sheet 2 INVENTOR ATTORNEY L/Nmuv Two/4, 50

Feb. 6,1945. L. THOMPSON 2,369,088

VARIABLE RESISTANCE EQUALIZER 'Filed July 20, 1945 5 Sheets-Sheet s PRE- AMPLIFIER NEGATIVE/"5E0 BACK LOOP zmo RESIST/1N 5 25,0004'2 Res/sums AT INSIDE or RECORD 4T OUTSIDE OF REcaRD INVEIQTOR LINCOLN THOMPSON ATTORNEY Feb. 6,1945. 7 I L. THOMPSON- 2,369,088

VARIABLE RESISTANCE EQUALIZER Filed July 20, l943 5 Sheets-Sheet 4 400 crass AMPLITUDE 1 /500 crews AMPLITUDE= 7 COMP05/TE TkACK OF BOTH WA vs IN\IIENTOR LINCOLN THOMPSON ATTORNEY VARIABLE RES I STANCE EQUALI ZER- Filed July 20, 1945 5 Sheets Sheet 5 INVENTOR 'L/NCOLN THOMP50N ATTORNEY Patented F ch. 6, 1945 UNITED. STATES PATENT OFFl CE VARIABLE RESISTANCE EQUALIZER Lincoln Thompson, Cheshire, Conn., assignor to The Soundscriber Corporation, New Haven,

Conn., a corporation of Connecticut Application July 20, 1943, Serial No. 495,406

1 Claim. (Cl. 179-10014) This invention relates to phonographs and more particularly to a dictating machine employing sound recording and sound reproducing mechanisms.

One objector this invention is to provide a dictating machine of the above nature, in which the sound waves are recorded at a low speed in closely spaced grooves on a rotating disc record of the type described in my prior patent, Reissue 22,183 of September 22, 1942, entitled Apparatus for recording sound on thin disks.

A further object of the invention is to provide a dictating machine of the above nature for producing disc records, in which the sound quality is substantially constant throughout the length of the records.

A further object is to provide a dictatingmachine of the above nature, in which a degree of high frequencye-accentuation is utilized at the center portion of the record which is proportioned to the'trackspeed, the stylus diameter, the contour of the contact of the stylus with the side of the groove, the top frequency to be recorded, and the speech energy characteristics.

A further object is to provid a device of the above nature which will be simple in construction, inexpensive to manufacture, easy to install and manipulate, and very eflicient and durable in use.

With these and other objects in view there has been illustrated in the accompanying drawings one form in which the invention may be conveniently embodied in practice.

In the drawings,

Fig. 1 represents a schematic view of one form of the invention.

Fig. 2 is a top plan view of the recorder-reproducing unit.

Fig. 3 is a sectional view of the same on an enlarged scale taken along the line 33 off-Fig. 2. Fig. 4 is a sectional view on line'4l of Fig. 3 taken through the variable resistance unit to show the interior construction thereof.

Fig. 5 is a chart showing curves representing iable resistance in the negative feed-back loop thereof in series with a tuned circuit.

Fig. 8 is a diagram showing a record track formed by the recording stylus when vibrating at 400 cycles per second.

Fig. 9 is a. similar diagram showing the track produced by the stylus when vibrating at 600 cycles per second and with one-half the amplitude of the 400-cycle note. I

Fig. 10 is a diagram showing the composite record track which results when the 400-cycle and GOO-cycle notes of Figs. 8 and 9 are produced simultaneously.

Fig. 11 is a microphotograph of an actual record track of a SOOO-cycle note having a normal amplitude and with a, track speed of about 50 per minute.

Fig. 12 isa similar microphotograph of the record track of the same note recorded with a track speed of 30 feet per minute.

Fig. 13 is a microphotograph of the record track produced by a 400-cycle note with normal amplitude and with a track speed of 30 feet per minute. 7

This invention is an improvement over the invention disclosed in Patent No. 2,239,042 dated April 22, 1941, entitled Wave recording and re-- production," issued to Kleber and Thompson.

. In order to compress as much recording as possible in a given length of record track, it is desirable to record th grooves at a relatively low speed. Dimculties arise, however, at the inner portion. of a spirally grooved record, because of the fact that the record is exceedingly'compressed and suffers serious loss of the higher frequency.

components of the sound when reproduced. At the higher recorded frequencies and these lower speeds, the reproducing stylus is unable to follow faithfully "the fine structure of the recorded waves representing the components of such higher frequencies. When recorded with their normal value the result is that the reproduced waves are correspondingly distorted and incomplete.

By means of the present invention the above and other disadvantages have been avoided, and

.a dictating machine has been produced which will compactly record speech sound waves in such a manner that they may be reproduced with a minimum distortion, loss of identity and mutilation, and with substantially constant quality throughout the course of the record.

In accordance with the present invention. the high frequency components-of the sound waves at the lower track speeds are scientifically compensated or accentuated during the recording so that are transmitted t the recording head at higher power levels than they would otherwise have whereby the reproduction will be more faithtake place in progressively increasing amounts for the inner portions of the spiral groove with their correspondingly decreasing track speeds by recording the higher frequency components with amplitudes increasingly greater, and in greater proportion to those of the lower frequency components.

As herein disclosed, the recording amplifier is I provided with a negative feed-back circuit, in-

cluding a variable resistance, which is controlled by the movement of the recording arm so that the amplification of the higher frequency components will be relatively greater at the inner portion of the record than that of the lower frequency components. This is due to the fact that the variable resistance together with a'series tuned circuit comprise a shunt across the feedback loop so that for the frequency at which the feed-back circuit is tuned (3000) the impedance of said circuit will approach zero and the impedance of the whole shunt circuit will approach that of the variable resistance alone. This will cause the negative feed-back loop to be shortcircuited except for the variable resistance. It will be understood that short-circuitlng the feedback loop eliminates the feed-back and gives the increased amplification normal to the amplifier when operating without feed-back. Thu when the variable resistance is low, and the tuned circult is in resonance, the negative feed-back loop will be almost completely short-circuited and the electron tube circuit will operate at full amplification. At other frequencies than resonance the short circuiting will be less in proportion to the total impedance of the tuned circuit for the'fre quency in question. Q I

As herein disclosed, the variable resistance is mechanically actuated by mean of a gearing arrangement connected with the constant speed recording arm. Both the rate of change of this resistance with the arm position and the shape of the compensating curve are important in proto a recording amplifier l2, the output of which is transmitted by a cord II to a recording head 22 of a recorder-reproducer unit l4. The re corded sound wave currents are transmitted from a reproduce;- pickup head 28 through a cord II to a reproducer-amplifler l6, which in turn is connected by a cord II to aloud speaker I 8.

Mounted upon the base IQ of the recorderreproducer unit I4 is an angular recording arm (see Fig. 2), which is yieldably driven at constant speed by a vertical feed shaft 2| (see Fig. 3), receiving power from a motor, not shown. A thin disc sound record 23, preferably made from plastic material such as Vinylite, is mounted on a rotating turntable 24 under the recordinghead 22, which ispivotally mounted in the end of the recording arm- 20. The record 23 is centralized by and interlocked with aspindle 25' located at the centerof the turntable. .The sound record i thus embossed on the record 23 in the'formof an undulatingspiral groove or track.

The record is adapted to be transcribed by 7 means of a reproducing arm," mountedto re- Amplifier circuits The sound wave currents from the microphone in are flrstpassed through a pre-ampliner 30 which is coupled to the primary coil 3| of a transformer am, the secondary 32 of which is located in the input circuitof the amplifier, said through ducing the type or degree of fidelity required from the recording-reproducing process. 0 Althoughthe fidelity of dictation reproductio does not need to be equal to high fidelity musicalreproduction, it is desirable to maintain as high a fidelity as is consistent with the long playins time of the record so as to maintain intelligibility and thus facilitate the transcription of the record by the typist. For this reason it is important that the frequency response shall extend over the average speech range (200 to 3000 cycles), and wave form distortion be reduced to the minimum; otherwise, the sounds will be- The reproduction of the recording made with the compensating high frequency accentuation toward the center of the record, by this method, will have substantially the same listening quality at all parts of the record, as the balance between thehigher and'lower frequencies will be malntainedeven though the track speed is indicates a microphone connected by a cord H come very unpleasant when listened to for a 1 long period of time.

circuit also including a, conductor 33 connected with the control'grid 34 of" an electron tube 35. The secondary 32 is also connected by a conductor 3511 ten fixed resistance 38 joined.

a conductor 36m: to a conductor 5| which in turn leads to a condenser .39 having a resistance '30 in shunt therewith, said condenser also beingv connected to the cathode 36a of the electron tube 35.

Theelectron tube 33 also contains a heated filament 36b and an anode or plate 31 which is located in the output circuit of the amplifier. The plate 31 is connected by a conductor 31a to the pr ry 40 of an output transformer, the secondary 4| of which leads through a pair of conductors" and '43 to the recording head 22. I

previously mentioned.

In order to control the amount of equalization or compensation of the high frequency components of the sound waves, provision is made of a negative feed-back loop circuit C, including a pair of conductors 44 and 45 in' series with which are an inductance coil 48, a condenser 41, and a variable'arcuate resistance 48 which is engaged by a movable contact 49, which sweeps across it. The contact 49 is connected with the conductor 43 by means of a pair of conductors 50 and 52.

As clearly shown in Figs. 2, 3 and 4', the variable resistance 4B is housed within a casing Bin and is embraced and supported by a bracket member 32a forming a bearing .for a vertical shaft 53 On the upper end of which is a pinion I 54; The pinion l4'meshes with a gear segment 55 mounted on the free end of the arm 58, the

latter being secured rigidly to the rear end of the recording arm 20, a by a screw 51. The

rear end of the recording arm 20 is provided with a circular aperture 56a surrounding the feed shaft 2|, and said arm is adapted to be yieldably driven by means of a power head 58, rigidly -ing response at the inside of the recording stylus and a normal speed connected to said shaft by a pin 59, said head having frictional engagement with the under side of said recording arm 20.

The chart shown in Fig. indicates by curves 60, 6| and 62 how the compensating response of the high frequency wave components varies as the recording stylus approaches the center of the record. The curve 60 represents the compensatthe record, the curve 6| the response at the intermediate portion, and the curve 62 the response at the outside of the record respectively.

Fig. 6 illustrates a set of curves 63, 64 and 85, obtained by reproducing a recording made at the same three points on the record, showing the substantial equalization produced by applying compensation in recording to the high frequency wave components at the inner portion of the record, the dotted line curve 66 indicating what the reproduction at the inner portion of the record would be without compensation.

The practical limit to the amount of high frequency compensation which can be used at the low track speeds at the center of the record is determined by the fact that the physical size of point becomes greater in comparison to the small dimensions of the high frequency waves being recorded as the track speed becomes lower. I speed approaches a standstill, the groove will merely be widened without showing any modulations. Conditions intermediate this extreme are illustrated by the diagrams shown in Figs. 8, 9 and 10.

Fig. 8 shows the groove produced by a 400- cycle wave,'and Fig. 9 the groove of a 1600-cyole Wave of one-half the amplitude of the wave shown in Fig. 8. These two waves are combined in Fig. 10 to show the groove of the resultant composite wave.

In ordinary speech a 1600-cycle wave and higher components will normally have an amplitude much lower than one-half that of the 400-cycle wave. This is due to the well-known fact that most of the energy in speech is located in the range of sound waves below 600 to 800 cycles. The energy of the high frequency waves is further reduced because the amplitude of a wave varies inversely with its frequency for a given energy content. At higher frequencies (such as 3000 cycles) the accentuation can even be as much as thirty times thatat 400 cycles with a track speed of 10 to feet per minute.

The microphotograph shown in Fig. 11 indicates the undulations of a record groove of a 3000-cycle wave recorded with normal amplitude, at a track speed of about 50 feet per minute. Fig. 12 shows a similar microphotograph of a 3000-cycle wave recorded with approximately the same amplitude at a track speed of 30 feet per minute. Fig. 13 shows a microphotograph of a 400-cycle wave recorded with normal amplitude at 30 feet per minute. It will be noted that the undulations are substantially perfect with the 300 0-cycle wave recording normally at 50 feet per minute, but with the 3000-cycle wave recorded at the same amplitude at a track speed of 30 feet per minute, the widening and scalloping effects on the groove are clearly shown. At lower amplitude, these effects are lessened for a given tracl; speed, and they would also be lessened by asmaller stylus point.

With a given stylus point it is obvious that with a fixed amplitude, the'track speed that can be used for a given high frequency wave is 111m- For example, if the track' ing from its spirit.

ited, or if the lowest track speed is also fixed, the highest frequency for satisfactory recording is determined.

The above reasoningapplies to the recording of single frequencies, but in actual voice recording of sounds having mixed waves of many frequencies, the higher frequency components, while very important to intelligibility, fortuantely are low in energy content, and can be accentuated in the recording without harmful groove effects, to a far greater degree than is indicated by the analysis of single frequencies. Obviously it -is undesirable to record any frequencies higher than those necessary-to obtain good intelligibility, and an excess of higher frequencies above those necessary will merely serve to produce undesirable groove enlargements, scalloping effects and consequent distortions. It will thus be seen that with the apparatus herein disclosed the use of a tuned circuit in the feed-back loop produces maximum amplification at the tuning frequency point, and amplification of the still higher frequencies progressively falls off as the frequency increases. over the results which would be produced merely by, a simple condenser in series with a variable resistance.

Operation In operation, as the recording arm moves uniform'ly from the outer portion toward the inner portion of the record where the lineal speed of the groove is smallest, the compensation of the feed-back circuit will progressively shunt the resistance 36, and the amplifier l2 will be given a predetermined distortion characteristic caus-' ing a selective increase in the amplification 01' the higher frequency components.

While there has been'disclosed in this specification one form in which the invention may be embodied, it is to be understood that this form is shown for the purpose of illustration only, and that the invention is not to be limited to the specific disclosur but may be modified and embodied in various other forms without depart- .In short, the invention includes all the modifications and embodiments coming within the scope of the following claim. Having thus fully described the invention, what is claimed as new, and for which it,is desired to secure Letters Patent, is:

,In a system for recording speech and other sounds on a disk record, a microphone, an electron tube amplifier connected with said microphone and including input and output circuits,

,said input circuit having an adjustable inverse pinion meshing with said sector gear, a central shaft for said pinion, and a swinging contact arm on said shaft, said contact arm engaging said variable resistance to automatically vary the when said recording arm is swung inwardly across the record.

' LINCOLN THOMPSON.

A great advantage is thus obtained 

